Method for forming complex composite articles

ABSTRACT

A method of making a flexible composite tool particularly adapted for composite molding and the composite tool made by this method. The method includes dissolving a B-staged fluoroelastomer in a suitable solvent such as methyl ethyl ketone or toluene to form a fluoroelastomer solution. A fabric is then coated with the fluoroelastomer solution. The solvent is evaporated and the impregnated fabric is sandwiched between unreinforced sheets to form a reinforced tooling material. The reinforced tooling material is laid up with alternating layers of unreinforced fluoroelastomer and then cured to form a composite tool which is stable through multiple thermal cycles. In addition, an elastomer tool can be provided with detail cavities shaped to accept separately formed detail structures, with each cavity including extra reinforcement in order to locate the details to close tolerances without extensive hand positioning or use of adhesives. The tool is useful in molding composites and is reusable. The life of the tool is further prolonged by a described method of repairing damaged portions of the tool.

This invention was made with Government support under a contract awardedby the Department of the Army. The Government has certain rights in thisinvention.

This application is a division of application Ser. No. 07/576,176, filedAug. 30, 1991, now U.S. Pat. No. 5,071,338, which is a continuation ofapplication Ser. No. 07/128,134, filed Dec. 3, 1987, now abandoned,which is a continuation-in-part of copending U.S. patent applicationtitled, "Solution Coating for a B-Staged Polyaramid Fiber ReinforcedFluoroelastomer Tooling Material", Ser. No. 093,937, filed on Sep. 8,1987, now abandoned.

TECHNICAL FIELD

This invention relates to molding of composites, and more particularly,to the tooling and methods for forming integral composite articlesrequiring co-curing with separately formed detail structures.

BACKGROUND ART

There are various methods of molding composites. One method involvesmatched metal molds in which two halves of a mold are pressed togetherand heated (cured) to form an article. Although this can be a veryaccurate method, the cost of manufacturing the matched metal molds isvery high, making the process uneconomical for small production runs. Inaddition, slight inaccuracies in the layup can cause large variations inthe properties of the cured composite. Another method which has hadwidespread use is vacuum bag molding. A thin film, typically nylon, isplaced over the article to be molded, forming a bag type enclosure, anda vacuum is drawn on the bag prior to heating the assembly to cure thecomposite material. For high quality material, the assembly is placed inan autoclave and external pressure is applied during the cure. While thevacuum bag molding method is lower in cost and more tolerant to materialvariations or layup inaccuracies than the matched metal molding method,the bags tend to wrinkle and cause the molded article to have an unevensurface. Also, the vacuum bag can catch on high points in the layup andbridge, or fail to contact the entire surface. Bridging results ineither improper compaction or stretching of the bag beyond its yieldpoint with resultant failure of the bag. In either case, the propertiesof the cured material are degraded. Finally, there is a great deal oflabor involved in placing the bags over the composite articles andsealing them, and the bags can only be used once.

Another type of vacuum bag is made from silicone rubber, eitherreinforced, or unreinforced. This bag is designed to be reusable,however, the silicone materials tend to interact with the curing agentsused in the composites and they become brittle. This results inrelatively short lifetimes.

Accordingly, there has been an ongoing search in the art for a reusabletooling material which is flexible enough to conform to the part surfaceand provide uniform cure pressure, and yet sturdy enough to withstandthe rigors of material handling and chemical interaction with the curingresin.

In forming integral complex composite structures, which requireco-curing of previously produced detailed structures such as structuralsupports or struts, problems occur in locating the details on thecomposite sheets and in preventing movement during compaction andcuring. Generally, a composite material, usually a preimpregnated fiberfabric or tape in sheet form, is layed-up in a female mold and thedetails positioned on the sheet material by hand. The details mayrequire physical structures, such as blocks and bridges, adhesives orother means to hold the details in place. A flexible bag is thencarefully laid over the mold and drawn down with vacuum to compact andhold the plies for molding. It is very difficult to locate the details,requiring templates and hand measurements to assure proper positioning.It is also difficult to prevent movement during the addition of the bagand handling of the mold, resulting in generally unacceptablepart-to-part reproducibility. Consequently, details are usually attachedto the article after molding using skin penetrating fasteners.Therefore, the search continues for a method for molding highly complexintegral parts with precisely located detail structures.

DISCLOSURE OF THE INVENTION

This invention is directed to a method of making a flexible compositetool particularly adapted for composite molding and the resulting toolwherein the tool is reusable. The method comprises dissolving B-stagedfluoroelastomer in a suitable solvent to form a fluoroelastomersolution. A fabric is then coated with the fluoroelastomer solution. Thesolvent is evaporated and the coated fabric is calendered or rolledbetween two unreinforced sheets to form a reinforced tooling material.The reinforced tooling material is laid up over a model of the desiredshape and cured to form a composite tool which is stable through aplurality of thermal and chemical cycles.

Another aspect of this invention relates to molding composites using thedescribed reusable tool. Prepregs or fiber reinforced polymer sheets arelaid up on the reusable tool. Pressure and heat are applied to cure theprepreg or sheets and form the composite. The composite is removed andthe tool is ready to use again. It is estimated that the tool will lastthrough at least 50 to 100 cycles (instead of the 1 cycle forconventional vacuum bags) and if the tool becomes damaged, a method ofrepairing the damaged portion is described to further prolong the lifeof the tool.

Another aspect of this invention relates to producing a semi-rigid toolfor forming a complex composite article, accurately locating separatelyformed detail structures on the article for co-curing therewith. Thesemi-rigid tool comprises an elastomer layer formed to a shapeessentially matching the shape of the article, thereby forming a cavityabout each detail structure. The tool further includes at least oneadditional layer of a reinforced elastomer placed about each cavity, theadditional reinforcement increasing the rigidity of the elastomer layerabout the detail cavity, preventing moving or shifting of the detailduring processing. The cavities which are formed to match the separatelyformed details provide pockets shaped to accept placement of the detailin the tool prior to molding. This assures proper alignment of thedetails without physical structures or exhaustive hand layup.

In yet another aspect of the present invention, a method is disclosedfor molding a complex composite article which requires accuratelyincorporating separately formed structures in the composite article. Themethod comprises providing a tool which includes a semi-rigid tool halfcomprised of an elastomer layer formed to a shape essentially matchingthe shape of the article, thereby forming a cavity about each detailstructure. The tool half further includes at least one additional layerof a reinforced elastomer placed about each cavity, with the additionalreinforcement increasing the rigidity of the elastomer about thestructure, preventing moving or shifting of the structure duringprocessing. The complete tool also includes a rigid tool half shaped tomate with the semi-rigid half. The next steps involve laying up acomposite material on the rigid tool half, inserting the separatelyformed structures in the cavities of the semi-rigid tool, mating the twotool halves, and, processing to form the final article.

In a preferred embodiment of the present invention, the semi-rigid toolcomprises a fluoroelastomer sheet impregnated polyamide fabricsandwiched between unreinforced fluoroelastomer sheets, with the sheetslayed-up in a shape to match a finished article and then cured. The toolfurther includes at least one layer of reinforced fluoroelastomer placedabout each detail cavity for preventing shifting of the details duringmolding.

Other features and advantages of the present invention will becomeapparent in light of the following description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a finished article including integrallymolded struts and supports.

FIG. 2 is an illustrative view of a typical semi-rigid elastomer toolincluding cavities for receiving detail structures.

FIG. 3 is an enlarged sectional view of a typical detail cavityincluding additional reinforcement.

FIG. 4 is a front view of a complete mated tool.

FIG. 5 is a side view of a mated tool with the strongback removed.

FIG. 6 is a front view of a rigid tool half.

BEST MODE FOR CARRYING OUT THE INVENTION

The fluoroelastomer solution comprises a B-staged fluoroelastomer and asuitable solvent. "B-staged fluoroelastomer" means a fluoroelastomerwhich has been partially cured, may be chain extended, but is notcross-linked. A B-staged fluoroelastomer is flowable and stretchable,but is not tacky.

The fluoroelastomer is made of fluoroelastomer resin, chain extenders,reactive and/or nonreactive fillers, and curing agents which will reactwhen the compound's temperature is raised. Fluoroelastomers arepreferred because of their inertness and high use temperatures.Inertness is preferred in the molding process to reduce the reactionbetween the resin that is being molded and the tooling material.Fluoroelastomers are also preferred because they tend to have higher usetemperatures than their bromine or chlorine analogues. Most preferredare fluoroelastomers which produce a rubber with a hardness of about 70durometers to about 80 durometers because of its intended application asa flexible tool. It is believed that any halogenated elastomers,fluorosilicone elastomers or fluorophosphazene elastomers could be usedin the practice of this invention. Exemplary materials are Fluorelfluoroelastomer supplied by 3M Company, St. Paul, Minn.; and Vitonfluoroelastomer manufactured by E. I. DuPont DeNemours Corporation,Wilmington, Del.

A suitable solvent must be chosen. The solvent must dissolve thefluoroelastomer completely, which means that there is no fluoroelastomerprecipitate and that higher weight polymers are not left undissolved.Methyl ethyl ketone (2-butanone) and toluene (methylbenzene) arepreferred solvents, but typically any alkyl aromatic solvent can also beused. Typically about 80% by volume to about 99% by volume of solvent isused. Preferably, the fluoroelastomer solution comprises about 90% toabout 95% by volume solvent. If too little solvent is used, then thesolution is too viscous and will not properly impregnate the fibers,while if too much solvent is used, then very little fluoroelastomer isimpregnated into the fibers and more impregnation cycles are required.The solvent serves two functions. It dissolves the fluoroelastomerallowing impregnation and also washes any residues of the cleaningprocess from the fiber. Without the final cleansing action of thesolvent, the fluoroelastomer may not stick to the fiber.

Typically, the fluoroelastomer compound used comprises thefluoroelastomer resin and several additives which may include agents(such as dicyanamide or bisphenol A), fillers (such as carbon black), orscavengers (such as magnesium oxide or calcium oxide).

Polyaramid fiber such as Kevlar fiber, commercially available from E. I.DuPont DeNemours, Wilmington, Del., and other companies, is thepreferred fiber to use in the invention. However, other fibers could beused also, such as graphite or glass fibers. Polyaramid fibers arepreferred because of their high tear strength properties. Polyaramidfibers are also a good choice because of their high tensile propertiesincluding both strength and modulus.

Typically, the fluoroelastomer solution is coated onto a fabric, whichis a plurality of interwoven bundles of fibers. Typically, for thisapplication, each bundle comprises 6000 fibers. Typical fabrics, usedfor this application, have between 10 and 15 bundles per inch in thewarp and fill directions. An exemplary fabric is Clarkschwebel TextileCompany's, style 354, which is a symmetrical basket weave material with13 bundles per inch in both directions. Various styles of fabric anddeniers of yarn can be used. The fabric normally has approximately thesame strength in both the warp and the fill directions (parallel to theorthogonal fiber axes), although other weaves can be used. Elongation ofthe material is very low in the warp and fill directions, but high inthe bias directions.

In this process chosen fabric is impregnated with the fluoroelastomersolution. In the impregnation process, the fluoroelastomer solution maybe applied to the fabric in a variety of ways. For example, thefluoroelastomer solution could be brushed onto the fabric with aninstrument similar to a paint brush, or a series of rollers could beused to roll the fabric through a trough to coat the fluoroelastomersolution onto the fabric. Other methods which could be used toimpregnate the fabric could include pressure impregnation, where aseries of nozzles would be used to force the solution into and throughthe fabric. Another method, which would work with a more viscoussolution, would be to use a doctor blade to apply a uniform thickness ofthe material and a series of rollers to work the material into thefabric.

It is preferred that the polyaramid fabric be completely impregnated bythe fluoroelastomer solution. If not, the bare polyaramid fabric can actlike a wick and pick up water. Preferably polyaramid fabrics should bekept dry. It is preferred that every fiber is totally encapsulated byfluoroelastomer. If the Kevlar fabric is not dried before theimpregnation process, or the fibers are not completely coated, theaccumulation of water generated by the condensation reactions occuringduring curing can cause delamination of the layered structure andsubsequent failure of the tool made from this material.

The solvent is evaporated using conventional methods. The solvent iseither flashed off at elevated temperatures (but below that required tocure the fluoroelastomer) or evaporated at room temperature over alonger period of time. Preferably all of the solvent is evaporatedbefore encapsulation of the impregnated fabric between the layers ofunreinforced fluoroelastomer takes place or the trapped solvent mayvaporize during the cure process and cause failure of the material.

After the fabric has been impregnated with the fluoroelastomer andencapsulated with fluoroelastomer, the material can be fabricated into atool. First, reinforced tooling material is fabricated by sandwichingthe impregnated material between thin cover sheets of unreinforcedmaterial. Typical methods of sandwiching these layers would becalendering (rolling) or pressing. No adhesive is needed which isespecially beneficial because adhesive is subject to thermal degradationand could subtract from the life of the tooling material or subsequenttool. Second, alternating layers of the reinforced and unreinforcedmaterials are laid up over a model of the shape that is desired.Finally, the layup is cured to form a tool (such as a semirigid locatorcaul). The curing process takes place in two phases, cure and post cure.The cure phase is typically at about 300°-400° F. for about 3-5 hours atabout 100-200 psi pressure. The post-curing phase is typically at about400°-500° F. for about 8-10 hours at atmospheric pressure with norestraint. During the cure and post cure, hydrogen fluoride and waterare evolved. Although these are in low concentrations, it is preferredthat acidic residue on metal tooling after the cure cycle is removed.

Semi-Rigid Tooling

In forming a semi-rigid tool for providing articles requiring preciselocation of separately formed details, a model is first constructed tomatch the shape of the desired finished article. For example, referringto FIG. 1, a composite helicopter fuselage section 1 is shown whichrequires a smooth external skin and internally located struts andsupports. It is desirable to produce an integral structure usingseparately manufactured details to limit complexity in forming the finalfinished article. In addition, this allows integral molding ofstiffeners and bulkhead attachment points, avoiding the use of fastenerswhich require skin penetration.

A model of the fuselage section may be made of any suitable material andcoated with a mold release agent. The elastomer tooling material is thenlayed over the model with various strips cut and bonded around eachdetail. In a typical tool, about two layers of reinforced and two layersof unreinforced material may be alternatly used to form the main sheet,covering all detail and nondetail areas. For illustrative purposes, thefluoroelastomer material previously disclosed will be discussed indetail. However, it will be understood by those skilled in the art thatany elastomer material may be used. For example, silicone, neoprene ornitrile rubbers may be acceptable substitutes. In terms ofreinforcement, glass, kevlar or graphite fiber may be used. In chosing asuitable material, consideration must be given to providing stiffnessand rigidity in detail areas while allowing flexibility and stretch toprovide uniform compaction of the laminate plies. Of course,compatibility with the composite system and temperature limitationsshould be considered. For illustrative purposes, the semi-rigid toolhalf is comprised of a reinforced fluoroelastomer impregnated polyamidefabric sandwiched between unreinforced fluoroelastomer sheets. Eachsheet of unreinforced material may be about 0.030 inches thick and eachreinforced sheet about 0.040 inches thick, with the reinforced sheetincluding kevlar fibers at a ±45° orientation to the long axis of thetool. Of course, the material thickness and fiber orientation will bedetermined by the user.

After the layers are added to the model, a temporary vacuum bag is addedand vacuum applied. The elastomer plies are then drawn down to ensureprecise forming around the detail structures while consolidating thelayup. After the initial sheets are added, additional reinforcingmaterial is included about the detail structures to add rigidity inthose areas. This prevents the collapse or distortion of the detailcavities. For example, four more layers of material may be layed abouteach detail, two reinforced and two unreinforced, with each layertapered to provide a gradual buildup of material.

Referring to FIG. 2, a semi-rigid elastomer tool 2 shaped to complimentthe fuselage section 1 is shown, including cavities 3 for receivingdetail structures 4. Referring to FIG. 3, the eight ply reinforcedstructure incorporated about a typical detail is shown. Four base pliesincluding two reinforced 5 and two unreinforced 6 are overlaid with fouradditional plies in the detail area, two reinforced 7 and twounreinforced 8. It should be noted that excessive rigidity is to beavoided as an overly rigid area will become self-supporting and propercompaction and consolidation of the composite will not be achieved. Byincreasing the rigidity of the material about the details, withouteliminating the elasticity of the elastomer, precise incorporation andlocation of the details in the final structure is achieved.

It may be desirable to incorporate a plurality of pulling blocks aboutthe fluoroelastomer tool for actuating disengagement from a formedarticle. Positioning blocks may be located between the elastomer layers,and may comprise drilled and tapped aluminum blocks, anodized and primedto give a bondable surface. After complete lay-up, the sheets arecompacted by vacuum and cured as previously described. In addition, ithas been found beneficial to age the fluoroelastomer by temperaturecycling after coating with a mold release agent. Such aging preventssticking of the tool to the part. The finished semi-rigid tool includesa plurality of cavities or pockets shaped to match the separately formeddetails which will be co-cured to the composite skin material duringfinal production. Utilizing cavities in the semi-rigid tool assuresprecise location of the details without additional measuring ortemplates.

The finished fluoroelastomer tool is then attached to a strongback whichprovides a framework for supporting the flexible tool during layup andmating with a matching tool half, assuring precise alignment between thetool halves. Referring to FIG. 4, a complete tool 9 is shown in themated condition, including a rigid tool half 10 and a semi-rigid toolhalf 11 supported by a strongback 12. For illustrative purposes, FIG. 5shows the semi-rigid tool half 11 mated to the rigid tool half without astrongback, with FIG. 6 illustrating the rigid tool half.

The strongback 12 may be rotatable, allowing the semi-rigid tool to besupported in a position where all the details are easily loaded intotheir respective cavities, and then rotated for mating with the rigidtool half. The strongback 12 may be composed of steel aluminum oranother suitable material. Referring to FIG. 4, the semi-rigid tool half11 is supported by the strong back 12 which includes a main frame 13 forattachment of a plurality of release actuators 14 for controllablylifting the elastomer tool after molding. Each actuator includes anextendible cylinder 15 which attaches to an eye bolt 16 which isthreaded into a positioning block 17 embedded in the elastomer, holdingthe elastomer to the strongback. The strongback also includes a subframe18 which contains a contour grid 19 for supporting the elastomer in itsapproximate molding shape while it is in the detail loading position.FIG. 2 generally shows the semi-rigid tool in a position for receivingdetails.

Prior to mating the tool halves, a mold release agent, such as MillerStevenson MS-142 is applied to the semi-rigid tool half and baked forabout 2 hours at about 350° F., with the procedure repeated severaltimes to age the tool. A composite material is then placed on the rigidtool half and the details inserted into the detail cavities in thesemi-rigid tool half. The composite material may comprise one or morelayers of a resin impregnated woven fiber sheet, commonly referred to asa "prepreg". The details may be preplied and preformed composite shapesincluding honeycomb core sections. Once loaded, the semi-rigid tool isrotated, aligned and mated with the rigid tool, as shown in FIG. 4. Itis interesting to note that no adhesives or other material are requiredto hold the details in position during alignment and positioning of thesemi-rigid tool. It appears that the flexibility of the elastomermaterial binds the detail structures, thereby holding the details whilethe tool is placed in position. The composite is then vacuum compacted,heated and cured, such as in an autoclave, with the elastomer toolingassuring even resin impregnation about the composite and preciselocation of details.

After curing is complete, the semi-rigid tool is peeled off the completearticle using the retractable actuators following a programed sequencein which each actuator is operated in sequence, to prevent damaging thedetails during removal. The sequence is a function of detail shape andeach article will require some experimentation to determine the propersequence for removal. It should be noted that, where possible, the draftangles of the detail structures should be adjusted to provide ease oftool removal. For example, using sloped rather than straight sides easestool removal.

EXAMPLE

50 grams of B-staged fluoroelastomer was dissolved in 9 ounces of2-butanone. The solution was coated onto both sides of a 6 inch by 6inch Kevlar sheet with a brush. The fabric was allowed to air dry for 30minutes at room temperature to evaporate the solvent. Another coating ofdissolved B-staged fluoroelastomer was brushed onto each side bydissolving 100 grams of B-staged fluoroelastomer in 9 ounces of2-butanone and applying the solution to the fabric. The fabric was againallowed to air dry for 30 minutes at room temperature to evaporate thesolvent. This material was then sandwiched between two unreinforcedB-staged fluoroelastomer sheets to form a test coupon.

For a full scale test, fifty yards of Kevlar polyaramid fabric wasimpregnated (with the above B-staged fluoroelastomer solutions) andevaporated using a series of rollers and troughs in 47 minutes in aconventional oven at about 175°-180° F. This process was repeated threetimes to ensure better impregnation. After evaporation the impregnatedfabric was sandwiched between unreinforced B-staged fluoroelastomersheets by calendering the sheets onto the fabric. No adhesive wasneeded. The reinforced tooling material and unreinforced fluoroelastomerwere alternately laid up over a model of the desired shape and cured at350° F. for 4 hours at 100 psi and then post cured at 450° F. for 8hours at atmospheric pressure with no restraint in a conventional oven.

The resulting solution coated fluoroelastomer impregnated fabric toolcan be used to mold composites. Typically, at least one layer of afabric which has been preimpregnated with a polymer material (prepreg)is laid up on the tool. Heat and pressure are conventionally applied tocure the preimpregnated fabric to form a composite. Other conventionallyknown methods for molding composites using this tool will be apparent tothose skilled in the art.

The solution coated fluoroelastomer impregnated fabric tool is animprovement over the adhesive coated fabric used previously. With theadhesive coated material, the adhesive was not compatible with thepolyaramid fabric reinforcement and the tools failed prematurely. Thefluoroelastomer impregnated fabric tool approach to manufacturingcomposite parts has several advantages over currently used vacuum bags.The tool is stable throughout a plurality of thermal cycles andtherefore is reusable. Testing has demonstrated that at least 50 partscan be produced from one tool, while conventional vacuum bags produceonly one part and are usable only once. It is estimated that at least100 parts will be obtainable from a tool before it must be replaced.Another advantage is that the tooling material is not seriously attackedby the amine curing agents found in conventional epoxy resins and themechanical properties of the fluoroelastomer material do not degradewith time. Minor damage to the surface of the tool can also be repairedby coating the damaged portion with a fluoroelastomer solution similarto that used to impregnate the fabric. This gives increased toolinglife. In addition, the tool accurately locates the internal details of acomposite part and gives improved surface definition to those surfacesof the part in contact with the tool. This leads to superior qualityparts. The increased reinforcement of the detail cavities pro idesprecise compaction while ensuring that details are located to closetolerances. In addition, production time is substantially reduced asdetails are put in the detail cavities rather than layed up on thecomposite material, assuring good part-to-part reproducibility. It hasbeen found that such details can be located to a tolerance of 0.004inches using the semi-rigid fluoroelastomer tooling material of thepresent invention. In addition, since no structures, peel plies oradhesives are needed, both labor and material requirements are reducedand part throughput is increased.

It should be understood that the invention is not limited to anyparticular embodiment shown and described herein, but that variouschanges and modifications may be made without departing from the spiritor scope of this concept as defined by the following claims.

We claim:
 1. A method for molding a complex composite article andaccurately locating a separately formed detailed structure on thearticle for co-curing therewith, the method comprising:providing asemi-rigid tool half having an elastomer layer having sufficientflexibility and stretch to provide uniform compaction of a plurality oflaminate plys, the tool half formed to a shape essentially matching theshape of the article, the elastomer layer having one or more cavitiesshaped to accept a detail structure therein, and means for preventingmovement or shifting of a detail structure located in the cavity duringprocessing, said means including at least one additional layer of areinforced elastomer disposed over the portion of the elastomer layerforming the cavity, the additional reinforcement provided by thereinforced elastomer layer increasing the rigidity of the elastomerlayer about the detail cavity, without eliminating the elasticity of theelastomer layer, providing a rigid tool half shaped to mate with thesemi-rigid tool half; placing the plurality of laminate plys on therigid tool half; inserting detail structures into the one or morecavities in the elastomer layer; mating the two tool halves; and,processing to form a composite article having accurately locatedintegral detail structures.
 2. The method of claim 1 wherein theelastomer layer comprises a fluoroelastomer impregnated polyaramidfabric.
 3. The method of claim 1 wherein the elastomer layer is a seriesof plies, comprising an impregnated polyaramid fabric ply sandwichedbetween un-reinforced fluoroelastomer plies.